By means of an insulating layer, an otherwise electrically conducting body can be insulated relative to a part of its environment. Above all, from the fields of magneto inductive flow measurement or pump manufacture, examples of such tubes are, per se, known to those skilled in the art. Conventionally, such pipelines, or measuring tubes, are composed of: a support tube, which is, most often, metal; an insulating layer; and a reinforcement provided in the insulating layer. The reinforcement can, in such case, be composed of a porous material. Usually, the reinforcement is introduced into the support tube and is in contact therewith. For this, most often, bronze spheres are sintered in the support tube.
An example thereof is the Promag H-series of Endress+Hauser having diameters smaller than 25 mm.
There are measuring tubes, especially those known from magneto inductive flow measurement, which are provided with a lining on the inside, in order to insulate the electrodes electrically relative to the support tube. This lining is usually called the ‘liner’. An adhered or vulcanized or pressed-in liner has the disadvantage, that the adhesion between support tube and liner in the case of temperature, or pressure, fluctuations is not sufficiently assured. This can lead to release, for example, due to different coefficients of thermal expansion between support tube and liner or through arising negative pressure, this meaning, thus, that the accuracy of measurement is significantly degraded. Sprayed on or thermally melted on liners have, most often, lesser insulating properties, whereby measurement accuracy and the lifetime of the measuring tubes decreases.
To the state of the art belong securement designs, wherein the liner is secured on the inside of the support tube in a dovetail configuration, or bridge, designs. Such a pointwise securement cannot durably prevent release of the liner. Furthermore, support bodies are known, which are introduced into the support tube, in order to strengthen the liner. Among such support bodies is a pipe, or tube, having a slightly smaller outer diameter than the inner diameter of the support tube and a certain number of holes. Such a pipe, or tube, is introduced into the support tube and is completely surrounded by the insulating material. The number of holes is chosen carefully, since, in the case of large hole separation, the danger of release of the insulation from the pipe serving as a reinforcement is present. The amount of insulating material needed to completely coat the pipe serving as a reinforcement is relatively high.
A further support body is constructed of a lattice- or net-like or three dimensional weave, which can be produced from various materials, for example, from metal, which is introduced into the support tube and connected by pointwise bonding therewith and which is surrounded by the material of the insulating layer. In order to assure a sufficient stability of the liner, many points of securement are necessary. Reinforcement with such a support body is in the case of a conical curvature of the inner diameter of the measuring tube very complicated to implement.
A further support body is composed of a porous material, into whose pores the material of the insulating layer partially penetrates and so clings to the support body. Such a support body is described, for example, in EP1039269A1. The support body is of bending resistant material, which should assure for the thus reinforced liner a sufficient stability against release from the support tube or against folding of the liner or against a collapse of the liner protection. The porous support body must for this purpose not contact the support tube or be securely connected therewith.
EP0581017B1 describes a measuring tube with a porous support body, which is in contact with the support tube and the material of the insulating layer does not reach the inside of the support tube. Thus, pores on the side of the support body facing the support tube are unfilled.
All these designs have the disadvantage, that they are relatively complicated to manufacture and, therewith, relatively expensive. A seamless connection between support body and support tube, especially a material bonded, interlocking connection, which can absorb axial and radial forces, is not provided.
For manufacture of a pipeline, or of a measuring tube, with an insulating layer, there are known to those skilled in the art, methods which secure a support body in a prefabricated support tube or manufacture the support body in the support tube and then introduce the insulating layer into the support tube bearing installed support body. EP1039269A1 describes one such method, wherein, in the lumen of a prefabricated support tube, a porous support body is formed exactly fitting, by introducing a sinter core into the lumen of the support tube, which forms with the support tube a sinter space, into which sinter starting material of the support body is introduced and sintered. Then, the sinter core is removed and, following that, then a casting core is introduced into the lumen of the support tube with support body, which forms with the support tube a casting space, into which the liquefied starting material of the insulating layer is charged, which penetrates into the support body and solidifies, after which he casting core is removed.
It has been found that the separate introduction of a support body means a considerable manufacturing effort, especially when such is to fit accurately and/or be fixedly connected with the support tube and/or when apparatus-related features, such as, for example, openings for later introduction of electrodes or coil cores in the case of magneto inductive flow measuring devices, must be taken into consideration.